The invention relates to a method for the encrypted radio transmission of data, in the form of short telegrams mutually spaced apart in time, between a transmitter and a receiver of a system, wherein the key can be changed and is derived from a current piece of information according to a cryptological method. This method can particularly be used in a tire pressure control system of a motor vehicle, wherein the data to be transmitted originate from a sensor provided in a vehicle tire and are wirelessly transmitted in an encrypted manner to a receiver provided at the vehicle body. With respect to the technical environment, reference is, for example, made to German Patent document DE 10 2008 038 680 A1.
In principle, an encrypting of telemetrically transmitted data is known, such encryption not only importantly protects against eavesdropping, but should also prevent the data receiver from receiving manipulated data or allow for the recognition of data as not originating from the assigned transmitter. Telemetric data transmission plays an increasingly significant role also in motor vehicles. Newer tire pressure control systems, for example, determine a pressure loss of a vehicle tire no longer from a comparison of the rotational wheel speeds of the different vehicle wheels and also no longer by way of an analysis of the vertical vibrations of the wheel determinable, for example, at the shock absorber, but rather by a current measurement of the air pressure in the tire, for which a pressure sensor is provided in the tire. These measured values can be transmitted in a relatively simple manner only by radio to an electronic monitoring unit, which is arranged at the vehicle body.
Currently, this radio transmission in tire pressure control systems takes place in the form of standardized so-called telegrams by way of an essentially defined radio frequency in an unencrypted manner. At most, a slight randomized displacement of the sending points in time of the telegrams may be provided here in order to avoid a continuous frequency cancellation. At least theoretically, in this case, by means of computer-controlled radio transceivers, the transmitted data could not only be tapped, but manipulated data could also be sent out and thereby enter the above-mentioned electronic monitoring unit of a motor vehicle. If these manipulated data were to point to a severe pressure loss, the driver of the motor vehicle would then be prompted to come to a stop. This could therefore theoretically be used for attacking the motor vehicle or its occupants. One remedy to be considered would be an encrypted radio transmission. However, customary keys lengthen the data quantity to be transmitted, which may basically be undesirable but, in the case of a tire pressure control system, may be particularly undesirable because the capacity of the battery provided in addition to the transmitter and the pressure sensor, as well as possibly additional sensors in the tire, for supplying these above-mentioned elements with electric power, would thereby represent an additional or increased load. The battery should, however, have a service life that is as long as possible.
An encryption for telemetrically transmitted data therefore is needed which does not result in any, at least in no significant, lengthening of the data telegrams to be transmitted.
For a method for the encrypted radio transmission of data according to the invention, this need is met in that the time period elapsed between a preceding telegram and the encrypted telegram implementing the desired data transmission is used as the key, this time period being the result of a value or amount of a parameter present in the system. Advantageous embodiments are described and claimed herein.
The present invention is based on the idea of not sending the above-mentioned telegrams in a fixed time slot pattern but determining the sending point-in-time within specific time slots by means of a cryptological process, so that the sending point-in-time itself represents the key information. Thus, the time interval between two mutually successive telegram emissions can be represented as follows:
Δt(m)=tTx(m)−tTx(n), wherein tTx(n) is the point-in-time at which the telegram having the number n was sent, and Δt(m)>0 is an essentially cryptological function of at least one parameter known in the system. Preferably, this characteristic quantity in the form of a value or amount of a parameter present in the system and the above-mentioned cryptological function Δt(m) are known in the transmitter as well as in the receiver because then the time period between a preceding sent telegram and a telegram currently to be sent or received can be determined in the transmitter as well as in the receiver from a precedingly transmitted value or amount of a characteristic quantity. However, when this preferably changeable parameter is known only in the transmitter, the time period can naturally also only be determined in the transmitter, so that this time period has to be wirelessly transmitted to the receiver for a future telegram. Although, by means of this information to be additionally transmitted, the data quantity to be transmitted is slightly enlarged, the expenditures for the transmission of this additional information is arguably low, specifically also when this additional information is transmitted in an encrypted manner according to a known method.
When applying the method according to the invention in a tire pressure control system of a motor vehicle, in which case the data to be transmitted originate from a sensor provided in a vehicle tire and are wirelessly transmitted in an encrypted manner to a receiver provided at the vehicle body, various parameters can be used for determining the above-mentioned time period. The value or amount, for example, of the pressure in the tire determined by the above-mentioned sensor can be used as a physical or electric parameter; in addition or as an alternative, the temperature in the tire determined by a sensor can be used. However, the appropriately determinable rotational speed and/or rotating direction of the tire or the vertical tire force on the road determined by way of a sensor or the residual service life of the battery provided in the tire can also be used. In each case, a combination of several parameters also is selectable. The same applies when the identification number (for example, of the pressure sensor) transmitted for the differentiation of the tires or a time period used as a key in a preceding transmission step is used as the characteristic quantity (or parameter).
For tire pressure control systems with a radio transmission of data, it is currently known to specify a so-called standardized radio telegram which is to be sent at least once per minute in the form of a base telegram with pressure and temperature information. This specification, which also applies to the manufacturer, ensures the compatibility for a minimum function, specifically for the pure pressure warning. Various manufacturers of motor vehicles transmit further data in addition to this base telegram, which data can be used, for example, for an automatic driving dynamics control of the vehicle. In the case of a tire pressure control system operating according to the invention, it may now be provided that these specified standardized radio telegrams are used only for marking the time synchronization, while manufacturer-specific telegrams with additional information and, particularly, with information encrypted according to the invention concerning the tire pressure and the relevant temperature between these standardized radio telegrams, are transmitted while applying the method according to the invention. In this sense, generally, i.e. not limited to a tire pressure control system, a preceding telegram can essentially be transmitted only for defining the beginning of the time period for the encrypted telegram carrying out the desired data transmission. Incidentally, the bit length of the key according to the invention can optionally still be extended in that, in addition to the bit information, which is transmitted by way of the sending point-in-time, a further bit sequence is added to the telegram. As a result, the bit length can be dynamically adapted to the protection needs of the data to be transmitted.
According to an advantageous further development, the above-mentioned time period is determined while additionally taking into account marginal conditions, specifically, with a view to how frequently the transmitted data or information are needed within a defined time period. Thus, for example, at higher driving speeds of the vehicle, a more frequent data transmission is desired than at a lower driving speed because, at least in the case of the conventional radio transmission, the yield of correctly received information is lower because of the cancellation effects at higher driving speeds. Furthermore, in the case of the latter, the probability of significant changes is also greater. In this sense, the determination of the above-mentioned time period between mutually successive telegrams can take place such that, as a function of the rotational speed of the tire, a shorter time period will be obtained at a higher speed than at a lower speed. When a dependence of the above-mentioned time period on the tire pressure is selected, a shorter time period may usefully be obtained when, viewed over successive telegrams, the change of pressure is relatively large, while a greater time period is obtained when, viewed over successive telegrams, virtually no change of pressure can be determined. As an alternative, the absolute value of the pressure in the tire measured by the sensor can be taken into account to the extent that shorter time periods are selected at relatively low pressure values and thus a greater number of telegrams are sent within a certain time period.
It may further be provided that misinformation that is received by the receiver in a time window that does not correspond to the time duration used as a key, is registered and stored as such. It can thereby be determined whether a targeted manipulation attack has taken place; i.e. whether misinformation was sent to the receiver in a targeted manner, in which case the driver of the vehicle can be correspondingly informed. In this context, by way of further antennas at the vehicle, the distance and the location of the source of the attack can also be estimated. In addition, by means of the above-mentioned conventional standardized radio telegram of a tire pressure control system, a position tracking spy system could prove by means of the reception of a transmitted identification number of a sensor that a certain vehicle was located at a certain time in a certain location and thereby violate the driver's privacy. With the method according to the invention, the probability of success of such a spy system is considerably reduced, specifically not only when, as optionally provided, fewer telegrams are sent at low driving speeds but particularly also because a one-time reception of an identification number is not sufficient for a sound reasoning that a vehicle was in a certain location, but several telegrams are required which have the correct time interval. This would not only increase the complexity of a position tracking spy system, but, particularly because of the progressive motion of the vehicle, it is not possible to receive several telegrams from a fixed antenna or a tracking system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.